Since cellulose fibers have advantages such as good dimensional stability, high adhesiveness, and a low temperature dependence of an elastic modulus (change in elastic modulus with respect to a temperature change), the cellulose fibers have been widely used in a tire as rayon.
However, since carbon disulfide is emitted in a process of producing rayon and the environmental load is very high, the cellulose fibers are not suitable for present day requirements aiming to produce products with raw materials that have a low environmental load.
The characteristics of the fiber material such as the above-described good dimensional stability, high adhesiveness, and low temperature dependence of an elastic modulus are significantly dependent on the fact that the fiber material is cellulose. Though synthetic fibers such as polyester and nylon are also used as a tire reinforcement cord, it is difficult to obtain the same degree of dimensional stability, adhesiveness, and elastic modulus as in the cellulose fibers.
Therefore, even though the environmental load is high, rayon is used in some tires at present.
In recent years, the environmental protection of the earth has been advocated, and thus, the use of cellulose that does not depend on fossil fuels as a raw material is desired. The use of carbon disulfide giving high environmental load in the production of rayon which is the above-described problem is needed in order to melt or dissolve cellulose when filming or fiberizing (spinning) cellulose.
In order to melt or dissolve a cellulose raw material, it is necessary to break intramolecular and intermolecular hydrogen bonds of three hydroxyl groups present in one repeating unit of cellulose. In the production of rayon, since it is possible to chemically modify the hydroxyl groups and break the hydrogen bonds by carbon disulfide, it is possible to melt or dissolve the cellulose raw material. In this manner, cellulose fibers obtained by spinning by chemically modifying the hydroxyl groups and regenerating the hydroxyl groups are generally called a regenerated cellulose.
As the raw material of the purified cellulose fiber, dissolving pulp having high purity can be used. The dissolving pulp having high purity is manufactured by purifying wood chips. Though the components included in wood depend on the species of wood, the components included are cellulose of 40% by weight to 60% by weight, hemicellulose of 10% by weight to 30% by weight, and lignin of 15% by weight to 30% by weight.
When a large amount of hemicellulose components is included in the purified cellulose fibers, strength tends to be decreased, and thus, the dissolving pulp having high purity is manufactured by purifying wood chips to remove lignin and hemicellulose. The proportion of the cellulose component in the dissolving pulp is 98% by weight or greater.
Furthermore, in a case of producing viscose rayon by using a viscose method, there is a tendency that a decrease in the molecular weight is severe in the step of dissolving cellulose, and by this, it is difficult to obtain a strong fiber. For this reason, in order to ensure high strength, it is necessary to use a material which does not include a low-molecular-weight component and has a proportion of the cellulose component of 98% by weight or greater.
Currently, the reason why cellulose fibers other than rayon are not widely used for tire reinforcement is that it is difficult to use a method by which high strength and break elongation are obtained when fiberizing cellulose, in addition to it being difficult to melt and dissolve a cellulose raw material by an industrially established method.
In contrast, according to a production method for purified cellulose fibers produced by using N-methylmorpholine-N-oxide (NMMO) as a solvent, it is possible to dissolve a cellulose raw material without chemical modification of the cellulose itself and without emitting carbon disulfide, and purified cellulose fibers obtained by dry-wet-spinning the cellulose using the cellulose solution produced by the method are excellent from the viewpoint that the environmental load is small and the chemically modified hydroxyl group does not remain (PTL 6).
However, purified cellulose fibers produced by using NMMO as a solvent do not satisfy the physical properties required for tire applications such as strength and break elongation.
In addition, in a case of using NMMO as a solvent, it is necessary to dissolve the cellulose raw material after swelling the cellulose raw material over a long period of time under vacuum distillation, and when excessively heated, there is a risk of explosion by a decomposition reaction of NMMO.
On the other hand, several types of the ionic liquids which efficiently dissolve the cellulose raw material have been reported (refer to PTLs 1 to 3). Dissolution of the cellulose raw material by the ionic liquid is due to solvation, and harmful substances such as carbon disulfide are not emitted in a process of producing purified cellulose fibers. Production of purified cellulose fibers is easily achieved by making a dissolved cellulose raw material pass through water, alcohol, or an aqueous solution of water and the ionic liquid. For this reason, in dissolution of cellulose by the ionic liquid, it is possible to suppress a decrease in the molecular weight of the cellulose (refer to PTLs 4 and 5). Spinning of cellulose using such an ionic liquid has been reported in PTLs 4 and 5. The production method for purified cellulose fibers obtained by using the ionic liquid can be said to be a method with a low environmental load.
In a tire, generally, the purified cellulose fibers are used as a fiber-rubber complex obtained by twisting the purified cellulose fibers to make a cord and coating the cord with rubber after an adhesion treatment. Fibers used in a tire are mainly used for the purpose of reinforcing rubber, and thus, fibers having high strength are preferable. It is possible to reduce the amount of fibers used in a tire as the strength of the fiber is high, and as a result, it is possible to reduce tire weight and rolling resistance.
Furthermore, by being able to reduce the amount of fibers used, it is possible to reduce the materials and energy required for tire production.